521 
FXUS66 KLOX 261007
AFDLOX

Area Forecast Discussion
National Weather Service Los Angeles/Oxnard CA 
307 AM PDT Mon Jul 26 2021

.SYNOPSIS...25/752 PM.

High pressure aloft will weaken through Monday prompting a 
cooling trend, but an easterly wave will bring increasing 
monsoonal moisture along with muggy conditions, especially on
Monday. Showers or thunderstorms are possible through Tuesday, 
especially on Monday when any area could see moderate to heavy 
showers with a slight risk of flooding.

&&

.SHORT TERM (TDY-WED)...26/251 AM.

A lot going on this morning. A Flash Flood Watch was issued for
the mountains of Los Angeles and Ventura Counties as well as the
Antelope Valley and the San Gabriel Valley, mainly in and around
the Bobcat, Dam, Ranch2 and Lake burn areas in the San Gabriel
valley foothills. This will be in effect through 8 PM this 
evening. Remnants of a huge thunderstorm complex that started 
around Las Vegas last evening continues to move west-southwest 
towards Southern California and latest radar indicated the leading
edge of the showers were already reaching eastern L.A. County. 
Embedded thunderstorms were occurring across San Bernardino and 
Riverside Counties, but look to be weakening as they move into
L.A. County this morning. However, there still could be some
thunder activity this morning into L.A. County over the next few 
hours. A site near Apple Valley around 130 AM in San Bernardino 
County received around 0.68 inches in 30 minutes. If these high 
rainfall rates with some of these showers continue to move across 
the recent burn areas mentioned in L.A. County, there could be 
some local damage due to shallow mud and debris flows. Once this 
large band of showers moves through the area this morning, skies 
will be partly cloudy and the combination of additional warming at
the surface, very juicy air (PWAT values around 1.75-2.00 inches)with
limited instability will set off more showers and thunderstorms, 
especially across the Los Angeles and Ventura County Mountains as 
well as the Antelope Valley, but also into Santa Barbara County 
and to a lesser extent across San Luis Obispo County. However, 
showers and thunderstorms could develop just anywhere today. 

Showers are expected to diminish later this evening, with the
possibilty of a few lingering showers into Santa Barbara and San
Luis Obispo County. 

For tonight/Tuesday, the upper level flow turns southerly which 
will push the monsoonal moisture north out of the area. There may 
be some coastal stratus tonight into Tuesday morning. Otherwise, 
will expect skies to be generally be mostly clear although some 
lingering mid level moisture could generate some clouds, and even
a slight chance of showers/thunderstorms Tuesday afternoon as
latest model soundings do indicate decent CAPE with some mid level
moisture over the LA/Ventura mountains and Antelope Valley Tuesday
afternoon. Temperatures on Tuesday will generally be 3-6 degrees 
warmer than Monday with ample sunshine and slightly higher 
thicknesses.

For Tuesday night/Wednesday, continued onshore gradients will
generate some stratus/fog across the coastal plain Tuesday night
and Wednesday morning. Otherwise, mostly clear skies can be
expected. As for temperatures, expect little change along the
coast, but about 2-4 degrees of warming across interior sections.

.LONG TERM (THU-SUN)...25/144 PM.

For the extended, models continue to exhibit good synoptic
agreement through the period. Main feature of note will be strong
upper level high that remains entrenched over the central part of
the country through next weekend. 

Forecast-wise, our area will remain on the periphery of the upper
level high. H5 heights will fluctuate from day-to-day with
continued onshore gradients near the surface. So, a shallow, but
rather strong, inversion should persist. Therefore, nighttime
stratus and fog will continue to impact the coastal plain and
lower coastal valleys. Given the upper level flow pattern, there
would be the potential for some monsoonal moisture to move into
the area Friday through the weekend. However, still much too early
to get any decent handle on this potential, so will keep with
mostly clear/partly cloudy skies for the area. As for the
temperatures, expect minor day-to-day changes with most areas
remaining a couple degrees above seasonal normals.

&&

.AVIATION...26/0110Z.

At 00Z over KLAX, the marine layer was 1200 feet deep. The
inversion top was at 2200 feet at 23 degrees Celsius.

Moderate confidence that a cluster of SHRA will move through the
area from east-to-west starting around 08Z over Los Angeles County.
Low confidence on if that cluster will have thunderstorms, but it
is possible. Isolated +RA IFR possible under heavier showers. Low
confidence on how this will impact the low clouds. TSRA is more 
probable over the mountains and KWJF KPMD after 18Z Monday. 

KLAX...Moderate confidence in typical MVFR low cloud regime
through about 08Z. Moderate confidence of SHRA OVC080-120 forming
in the 09-17Z period, with a 20 percent chance of brief IFR +RA
and/or TSRA with hail. Low confidence on how low clouds will 
react to the SHRA. No significant wind concerns. 

KBUR...Moderate confidence in VFR through about 08Z. Moderate 
confidence of SHRA OVC060-120 forming in the 09-17Z period, with 
a 30 percent chance of brief IFR +RA and/or TSRA with hail. No 
significant wind concerns.

&&

.MARINE...25/752 PM.

Isolated to scattered showers and thunderstorms could develop  
overnight and Monday. Any thunderstorm could produce local gale 
force winds and rough seas, dangerous lightning, heavy rainfall 
with reduced visibility, and waterspouts.

Patchy fog, locally dense at times, will continue into next week.
The poorest visibility and most widely spread conditions will 
occur in the overnight and morning hours along the Central Coast.

Winds across the outer waters will likely increase to Small Craft
Advisory (SCA) levels Wednesday night and Thursday. Otherwise and
elsewhere, conditions will remain below SCA level through at 
least Friday.

&&

.LOX WATCHES/WARNINGS/ADVISORIES...
CA...Flash Flood Watch in effect through this evening for zones
      53-54-59. (See LAXFFALOX).
PZ...NONE.

&&

.HAZARD POTENTIAL OUTLOOK (WED-SUN).

Monsoonal thunderstorms are possible starting Friday and lasting
through the weekend.

&&

$$

PUBLIC...Kaplan/Thompson
AVIATION...Kittell
MARINE...Kj
SYNOPSIS...Kaplan

weather.gov/losangeles